1. What is Positive Predictive Value (PPV)?

PPV is the probability that subjects with a positive screening test truly have the disease. It depends on the test's sensitivity and specificity, and the disease prevalence in the population.

2. How Does the Calculator Work?

The calculator uses the PPV formula:

Where:

• \( Sensitivity \) — True positive rate (0 to 1)
• \( Specificity \) — True negative rate (0 to 1)
• \( Prevalence \) — Disease prevalence in population (0 to 1)

Explanation: PPV increases with higher sensitivity, higher specificity, and higher disease prevalence.

3. Importance of PPV Calculation

Details: PPV helps clinicians interpret positive test results by showing how likely a positive result is to be a true positive. It's crucial for understanding a test's clinical utility.

4. Using the Calculator

Tips: Enter sensitivity, specificity, and prevalence as values between 0 and 1 (e.g., 0.95 for 95%). All values must be valid (between 0 and 1).

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between PPV and sensitivity?
A: Sensitivity measures how well a test identifies true positives, while PPV tells you the probability that a positive result is truly positive.

Q2: How does prevalence affect PPV?
A: PPV increases with higher prevalence. For rare diseases, even tests with high sensitivity/specificity may have low PPV.

Q3: What is a good PPV value?
A: Generally, higher is better. Values >0.9 are excellent, 0.8-0.9 are good, and <0.7 may be problematic depending on context.

Q4: Can PPV be calculated for negative results?
A: Yes, Negative Predictive Value (NPV) can be calculated with a similar formula focusing on negative results.

Q5: Why is PPV important in screening?
A: It helps balance the benefits of early detection against the harms of false positives, especially in low-prevalence populations.